Certain electronic devices may be supplied by a power source supplying a positive supply voltage. An audio signal generated by an audio device is a modulated signal with a rough mean voltage, called “common mode voltage”, that is generally positive, e.g. 1.2 V.
Headphones may be arranged so that the common mode voltage is equal to the ground voltage so as to ease driving of the headphones. If a speaker of such headphones receives an input signal having a common mode voltage different from the common mode voltage of the headphone, damage may occur to the speakers' cones.
For this reason, some audio devices may be configured for outputting an audio signal compatible with such headphones, that is to say, with a common mode voltage equal to the ground voltage.
Outputting a signal with a common mode voltage equal to the ground voltage may imply the use of both a source of negative voltage and a source of positive voltage. However, as already stated above, audio devices, especially in mobile devices, are generally provided with a power source that provides a positive supply voltage. A source of negative supply voltage may be desired. Such a negative supply source may include a negative charge pump, such as an inverter.
In certain charge pumps, a ground voltage is output to an output terminal in a first operating mode called charging mode, while electrical charge is stored into at least one capacitor. In a second operating mode called discharging mode, a negative voltage may be output by adequately coupling the capacitor to the output terminal. The charge pump thus outputs a pulse-width-modulated (PWM) signal, switching with a given switching frequency between the ground voltage output during the charging mode and the negative voltage output during the discharging mode. By coupling the output terminal of the charge pump to the input of a low-pass filter, adapted to filter the switching frequency, the PWM signal is averaged and the low pass filter outputs a continuous negative voltage.
The charge pump and the low pass filter form a charge pump assembly that is adapted to output a continuous filtered voltage. A −2 V negative voltage may thus be obtained.
However, there may be some challenges with such a technique. A charge pump may continuously use power when operating because a small current is dissipated in the succession of charging and discharging modes of the charge pump. For example, in the charging mode, some current is sunk to ground, while during the discharging mode, current is provided to a load as the load demands it.
Thus, the aforementioned small current, called “quiescent current”, may be dissipated even if no power is output by the charge pump assembly. The quiescent current of a charge pump depends both on the switching frequency and on the capacitance of the charge pump's capacitor.
Charge pumps are usually designed so as to be able to cope with unusual loads. Such unusual loads may be worst cases loads in which the headphone demands a relatively high current and a relatively high negative voltage from the charge pump. An unusual load may be used in audio devices when a loud sound is to be emitted from the speakers. Accordingly, the switching frequency and the capacitance of the capacitor of the charge pump are chosen to be high enough for the charge pump to be able to comply with demands for such unusual loads.
If the audio device is designed for supplying speakers adapted to produce loud sounds, it may desirably have a charge pump able to cope with unusual loads; accordingly the quiescent current of the charge pump is of interest.
A normal audio signal has an amplitude that is generally less than to 50% of the amplitude of worst-case signals. Thus, for most of the life of an audio device, the output audio signal is such that the audio device imposes on the charge pump assembly a load less than 50% of the worse-case load on the charge pump. Accordingly, the charge pump may be over-designed compared to its normal use.
As the quiescent current depends on worst-case assumptions of the load imposed on the charge pump, the quiescent current is of more interest than justified by a normal use of the audio device.
There is thus a desire for improved devices that reduce the power consumption of audio devices, and especially those that reduce the quiescent current.